Support unixgram syslog address

- Consider unixgram:// as a valid URL prefix
- Parse unixgram:// addresses
- Update docs

Signed-off-by: Zhuoyun Wei <wzyboy@wzyboy.org>

urlutil/urlutil_test.go

@@ -23,6 +23,7 @@ var (
 		"tcp+tls://example.com",
 		"udp://example.com",
 		"unix:///example",
+		"unixgram:///example",
 	}
 )